Regeneration of spent As-poisoned lead catalysts

ABSTRACT

Spent lead catalysts containing contaminating amounts of arsenic values, e.g., mixed oxides of lead and arsenic, are regenerated and rendered suitable for reuse by heat-treating same at a temperature of at least 550° C. in the presence of a gas mixture which comprises an inert gas, e.g., Ar, N 2  or He, and at least 2% by volume of at least one reducing gas, e.g., H 2 , CO or CH 4 .

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention:

The present invention relates to the treatment of spent catalystscontaminated or poisoned with arsenic in order to regenerate them andrender them suitable for reuse.

2. Description of the Prior Art:

The treatment of contaminated spent products is an activity which todayis becoming increasingly important. Environmental restrictions areresulting in the purification of all types of liquid, solid or gas withthe goal of diminishing their levels of pollutants and preventingdischarge of same to the environment.

Many catalysts have been developed for the purpose of removing theaforesaid impurities by absorption, adsorption or chemical reactions.However, once the contaminated products have been purified, theimpurities exist at high concentrations in the catalysts, which mustthen be stored or destroyed.

There currently is a tendency to treat these spent catalysts to removethe impurities therefrom. This treatment targets, on the one hand,recovering impurities, in order to profit therefrom or to convert theminto a form which can be easily stored, and, on the other, regeneratingthe spent catalysts in order to reuse them. This second aspect, inaddition, permits savings, since it is not necessary to repurchaseanother charge of fresh catalyst.

Arsenic plays an important role among the impurities which areparticularly harmful to the environment. In particular, it is present ingases, such as cracked gases resulting from steam cracking. One meansfor removing arsenic from these gases is to contact same with a catalystbased on a lead compound, for example a supported catalyst having anactive phase based on a lead compound, for example lead oxide.

The arsenic present in the gases reacts on contact with these catalyststo form an arsenic compound, in particular a mixed compound of lead andof arsenic, which remains ad/absorbed on the catalyst.

Spent catalysts comprising these arsenic compounds are presentlydestroyed or stored. In addition, a new charge of fresh catalyst must beemployed in order to continue to treat the gases to be purified.

SUMMARY OF THE INVENTION

A major object of the present invention is the provision of an improvedprocess for the treatment of spent catalysts of the above type whichmakes it possible not only to remove the arsenic values therefrom, butalso to regenerate the starting active phase of said catalyst,permitting reuse thereof for the same application without having toredeposit an active phase thereon.

Briefly, the present invention features a process for the treatment of asupported catalyst having an active phase based on a lead compound andwhich is contaminated with an arsenic compound, comprising heating saidcatalyst at a temperature of at least 500° C. in the presence of a gasmixture comprising at least 2% by volume of at least one reducing gas,the remainder being constituted by an inert gas.

DETAILED DESCRIPTION OF BEST MODE AND SPECIFIC/PREFERRED EMBODIMENTS OFTHE INVENTION

More particularly according to the present invention, featured is aprocess for the treatment of a supported catalyst having an active phasebased on a lead compound and which is contaminated with an arseniccompound, comprising heating said catalyst at a temperature of at least500° C. in the presence of a gas mixture which comprises at least 2% byvolume of at least one reducing gas, the remainder being an inert gas.

The reducing gas is advantageously selected from among H₂, CO, CH₄ orany other reducing gas.

The inert gas is advantageously selected from among Ar, N₂ or He.

The treatment temperature generally ranges from 550° C. to 1,000° C.,preferably from 700° C. to 900° C.

The residence time can vary widely. It preferably ranges from 1 to 15hours.

Similarly, the sweeping flow rate of the gas mixture can vary widely andpreferably ranges from 0.1 to 10 liters/hour/gram of catalyst.

The arsenic compound evolved during the treatment is removed in thegaseous form. These vapors can be recovered by any means known to thisart; for example, they can be condensed in the form of metallic arsenic.

The catalyst can be heated in any type of furnace, but a tubular furnaceor a stationary furnace is preferably employed.

The process according to the invention is particularly well suited fortreating a supported catalyst having an active phase based on leadoxide. The amount of this compound in the catalyst to be treated canalso vary widely. It can be as high as 22% by weight with respect to thetotal weight of the catalyst. This type of catalyst is poisoned with thearsenic in the form of a mixed oxide of lead and of arsenic.

The supported catalyst may include, for example, an inorganic supportbased on alumina, on silica, on silica/alumina or on charcoal.

For example, it can be the catalyst MEP 191 marketed by Procatalyse.

The removal of the arsenic is monitored by analysis using varioustechniques known for the quantitative determination of arsenic, inparticular atomic absorption.

A primary advantage presented by the process according to the inventionis that not only is the arsenic removed from the catalyst, but also theactive phase of the catalyst is regenerated and the support exhibits thesame characteristics as when it was used initially.

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

EXAMPLE 1

The starting catalyst was an MEP 191 catalyst marketed by Procatalyseexhibiting the following characteristics:

Support:

(a) alumina

(b) specific surface: 115 m² /g

Active phase:

(a) Pbo

(b) content: 23% by weight with respect to the catalyst

This catalyst was utilized in a unit for removing arsenic from a naturalgas. After the gas had been treated, the catalyst comprised 1.6% byweight of arsenic with respect to the total weight of the catalyst.

The catalyst was removed from the unit in order to be treated accordingto the process of the invention.

Procedure:

20 g of spent catalyst were introduced into a tubular furnace.

The temperature was increased to 100° C. at the rate of 5° C. per minutewhile sweeping the furnace with argon, introduced at a flow rate of 3liters/hour/gram of catalyst.

A gaseous mixture of argon and of hydrogen (2.5% by volume of hydrogen)was then introduced. The flow rate of the mixture was 3 liters/hour/gramof catalyst.

This treatment was carried out for 5 h at 700° C.

Results:

(i) content of arsenic: 0.14% by weight with respect to the catalyst,i.e., a removal of the arsenic of greater than 90%,

(ii) content of the active phase PbO: 22.4% by weight with respect tothe catalyst,

(iii) specific surface: 94 m² /g.

EXAMPLE 2

The procedure of Example 1 was repeated, except that the treatmenttemperature was 590° C. instead of 700° C.

Results:

(i) content of arsenic: 0.5% by weight with respect to the catalyst,i.e., a removal of the arsenic of 70%,

(ii) content of the active phase PbO: 21.2% by weight with respect tothe catalyst,

(iii) specific surface: 109 m² /g.

EXAMPLE 3 (Comparative)

The procedure of Example 1 was repeated, except that the treatment gascomprised only argon; no reducing gas was added thereto.

Results:

(i) content of arsenic: 1.46% by weight with respect to the catalyst,i.e., a removal of the arsenic of less than 9%,

(ii) content of the active phase PbO: 21.9% by weight with respect tothe catalyst,

(iii) specific surface: 76 m² /g.

EXAMPLE 4 (Comparative)

The procedure of Example 1 was repeated, except that the temperatureduring the treatment was 420° C. (less than 550° C.) and not 700° C.

Results:

(i) content of arsenic: 1.26% by weight with respect to the catalyst,i.e., a removal of the arsenic on the order of 20%,

(ii) content of the active phase PbO: 22.2% by weight with respect tothe catalyst,

(iii) specific surface: 108 m² /g.

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims, including equivalents thereof.

What is claimed is:
 1. A process for regenerating a spent lead catalystcontaining contaminating amounts of arsenic values, comprisingheat-treating said spent catalyst at a temperature of at least 550° C.in the presence of a gas mixture which comprises an inert gas and atleast 2% by volume of at least one reducing gas.
 2. The process asdefined by claim 1, said at least one reducing gas comprising H₂, CO orCH₄.
 3. The process as defined by claim 2, said at least one reducinggas comprising H₂.
 4. The process as defined by claim 1, said inert gascomprising Ar, N₂ or He.
 5. The process as defined by claim 1, carriedout at a temperature ranging from 550° C. to 1,000° C.
 6. The process asdefined by claim 1, carried out for from 1 to 15 hours.
 7. The processas defined by claim 1, comprising sweeping said spent catalyst with saidgas mixture at a flow rate ranging from 0.1 to 10 liters/hour/gram ofcatalyst.
 8. The process as defined by claim 1, further comprisingcondensing vapors of said contaminating arsenic values as metallicarsenic.
 9. The process as defined by claim 1, carried out in a tubularor stationary furnace.
 10. The process as defined by claim 1, said leadcatalyst comprising a catalytically active lead phase deposited onto aninorganic support substrate.
 11. The process as defined by claim 10,said catalytically active phase comprising lead oxide.
 12. The processas defined by claim 10, said inorganic support substrate comprisingalumina, silica, silica/alumina or charcoal.
 13. The process as definedby claim 1, said contaminating arsenic values comprising a mixed oxideof lead and arsenic.